The application relates to a novel foamable copolymer resin (i.e. StMMA) and a method for preparing the same, which resin is a new plastic molding material for producing eliminated pattern (lost foam) castings. The invention concerns the field of manufacturing lost foam casting molding materials.
From U.S. Pat. No. 5,403,866 is known a foamable resin composition which comprises a copolymer produced by copolymerizing 55-85% by weight, preferably 60-80% by weight, of styrene and 45-15% by weight, preferably 40-20% by weight, of methacrylic acid. The prior art document has some disadvantages, since tricalcium phosphate is used as main dispersant during the reaction and has to be treated with hydrochloric acid after the completion of the reaction, not only the difficulties of acid treatment are brought but also the amount of waste water is increased greatly; secondly, the foamable resin is prepared by xe2x80x9ctwo-step processxe2x80x9d and the foaming agent is added at a pressure in the middle of the reaction. Therefore, there are needed a plenty of apparatus investment and the operation is very complicated, resulting in a ery high production cost.
U.S. Pat. No. 6,360,807 discloses an expandable styrenic copolymer comprising from 50 to 90 percent by weight of methyl methacrylate and from 10 to 50 percent by weight of styrene. However, there are still defects present during the preparation of the copolymer resin by means of xe2x80x9cone-step processxe2x80x9d: the molecular weight is lower and the resultant resin foam (pattern) has a lower strength and therefore is easily deformable, so U.S. Pat. No. 6,360,807 has to make use of seed polymerization process. Therefore, the process is compocatedly operated, it is difficult to control the product quality, and the production cost and the plant investment are high.
In recent years, a lost foam (expandable pattern) casting (EPC) technology is increasingly used in casting technology, which is a new technique without residuals and with excellent moulding accuracy. Since a readily volatizable foam is used as mold pattern, it is unnecessary to take out the mold, any mold joint and any sand core are eliminated, the dimensional error caused due to the assembly of the core mold blocks is reduced. The dimensional accuracy and the surface roughness are comparable to the ones in investment casting. Since the eliminated pattern casting uses a dry sand molding process, the defects caused by water, adhesive and other additive present in the sand are eliminated and casting reject ratio is substantially reduced. Secondly, lost foam casting is easy to be performed cleanly. The pattern degradation when casting causes a little organic emission, which amounts to about 0.3% based on the weight of the poured molten iron, however accounts to about 5% in case of self-hardened sand. At the same time, the emission may be completed within a short time at a concentrated place. That is to say, the lost foam casting technique is also environmentally friendly. In addition, almost all the casting sand can be reused, for example up to more than 97%, omitting the operating system for the formulation molding sand and the recycle of the used sand. Thirdly, a lost foam casting allows a high degree of design freedom and an integration of several components into one casting, internal channels or openings can be included in the foam pattern to be cast directly, therefore machining cost and assembly cost can be reduced greatly. Generally speaking, by means of EPC process the equipment cost can be lowered by 30-50% and casting cost is reduced by 10-39%. Because of the possibility of carbon pickup, the process employing expandable polystyrene (EPS) pattern is not suitable for black metal casting since the carbon-rich benzene structure in EPS molecule is thermodynamically stable and difficult to be decomposed; Furthermore, the EPS decomposition leads to a plenty of liquid, which is contacted with molten metal for a long time, making carbon-hydrogen bond of benzene ring structure be cracked to form carbon residual and therefore increasing the possibility of carbon being captured by liquid metal in mold. In summary, the EPS mold has a main disadvantage of carbon pickup defect in casting.
The purpose of the application is to overcome the defects of the prior art, i.e. effectively solve the problem of a lower molecular weight of the foamable copolymer resin, obtain a product comparative to or superior to the foamable copolymer resin prepared by a seed polymerization or xe2x80x9ctwo-step processxe2x80x9d of the prior art, by selecting the constituting ratio of the mixed monomers and selecting one or more additives under the conditions that xe2x80x9cone-step processxe2x80x9d is employed in preparing process.
In eliminated pattern casting process, pattern manufacturing is very critical. Selection of starting materials and manufacturing process is very important for obtaining a pattern having a low density, an accurate size and a smooth surface and therefore for obtaining good castings. For this purpose, the applicant is trying to achieve the following results on designing the eliminated pattern casting molding material: 1. the evaporating temperature and the emitted gas amount should be low; 2. the evaporation should be carried out rapidly and completely without any residuals; 3. the materials should have low density, good strength and stiffness in order that the pattern maintains undamaged during its preparation, its transport and the filling of dry sand in mold, thereby ensuring the size and form of the pattern stable; 4. in order to solve the problems which are existing in preparing a foamable resin by means of xe2x80x9cone-step processxe2x80x9d, for example, that the prepared copolymer resin has a lower molecular weight and the resultant resin foam has a lower strength and therefore is easily deformable, the applicant has made a careful and extensive study and found that if divinyl benzene (DVB) or ethylene glycol dirnethacrylate (DMAE) or C8-C12-alkyl mercaptan or carbon tetrabromide or carbon tetrachloride as a modifying agent is mixed with the mixed monomers to carry out one-step polymerization, the resultant foamable copolymer resin product would have a greatly improved molecular weight, good performance and quality, leading to an excellent operabilityxe2x80x94with which one may produce a foam pattern having smooth surface and high strength, therefore obtaining excellent lost foam castings; 5. by selection of type and amount of the modifying aid in the composition, the constituting ratio of the polymerizating materials of the invention is fully different from the prior art, the resultant product has a different molecular structure and good suitability, meeting the need of the user.
It was found that the object of the invention can be reached by the following embodiments:
According to one aspect of the invention, there is provided a foamable copolymer resin which comprises in parts by weight (pbw) from 40 to 49 of methyl methacrylate, from 51 to 60 of styrene, from 180 to 220 of deionized water, from 0.25 to 0.55 of a dispersant, from 10 to 14 of a foaming agent, from 0.35 to 0.75 of an initiator and from 0.2 to 0.5 of a modifying aid.
According to another aspect of the invention, there is provided a foamable copolymer resin which comprises in parts by weight from 91 to 95 of methyl methacrylate, from 5 to 9 of styrene, from 230 to 270 of deionized water, from 0.4 to 0.6 of a dispersant, from 13 to 15 of a foaming agent, from 0.25 to 0.65 of an initiator and from 0.5 to 1.0 of a modifying aid.
According to further aspect of the invention, there is provided a foamable copolymer resin which comprises in parts by weight 70 of methyl methacrylate, 30 of styrene, from 180 to 220 of deionized water, from 0.3 to 0.6 of a dispersant, from 8 to 15 of a foaming agent, from 0.3 to 0.8 of an initiator and from 0.35 to 0.60 of a modifying aid.
In the above said embodiments, an organic peroxide may be used as an initiator, like benzoyl peroxide (BPO) and tert-butyl perbenzoatc (TBPB); the used foaming agent is pentane or isohexane or petroleum ether; the used modifying aid is divinyl benzene (DVB) or ethylene glycol dimethacrylate (DMAE) or C8-C12-alkyl mercaptane or carbon tetrabromide or carbon tetrachloride; The dispersant is cellulose ether like hydroxy ethyl cellulose or hydroxylpropyl cellulose ether etc. Preferably, the copolymer resin according to the invention comprises in parts by weight 49 of methyl methacrylate, 51 of styrene, 225 of deionized water, 0.45 of a despersant, 11.5 of a foaming agent, 0.55 of an initiator and 0.45 of a modifying aid.
Alternatively, the copolymer resin according to the invention preferably comprises in parts by weight 91 of methyl methacrylate, 9 of styrene, 250 of deionized water, 0.6 a dispersant, 15 of a foaming agent, 0.45 of an initiator and 0.8 of a modifying aid.
Alternatively, the copolymer resin according to the invention preferably comprises in parts by weight 70 of methyl methacrylate, 30 of styrene, 200 of deionized water, 0.35 of a dispersant, 12 of a foaming agent, 0.5 of an initiator and 0.65 of a modifying aid.
The new foamable copolymer resin according to the invention has the formula as follows: 
According to the present invention, there is also provided a process for preparing a new foamable copolymer resin, which comprises firstly adding an organic peroxide initiator, a modifying aid and a foaming agent to the mixed monomers of methyl methacrylate and styrene so as to constitute a homogenous mixture; at the same time charging the deionized water into a polymerization reactor and adding a dispersant with stirring to produce a homogenous mixture, then sealing the reactor and introducing N2 gas to purge the reactor before the above homogenous monomer mixture is added. The polymerization is carried out at a temperature of from 60 to 105 and a pressure of from 0.3 to 0.8 MPa for about 15 to 22 hours. After the completion of the reaction the residue is washed with water and dewatered, dried at a temperature of less than 40 to obtain foamable resin pellets. When drying the resin mixture a lubricant is added in an amount of not more than 1% by weight. The resultant copolymer resin has a particle size of 0.2 to 0.8 mm.
Compared to the prior art, the invention is different in that the foaming agent is added to the mixed monomers before the start of the polymerization and then they are together introduced to the reactor and in that several additives are introduced during the polymerization. The invention overcomes the defect that the polymer has a too low molecular weight, which is present in xe2x80x9cone-step processxe2x80x9d of the prior art. In the invention, the molecular weight of the copolymer may be controlled in a range between 150,000 to 300,000. Furthermore, The process of the invention has such advantages as: short procedure, easily operated, low production cost, reduced energy consumption and less apparatus investment. Since the constituting ratio of the polymerizating materials of the invention is fully different from the prior art, the resultant product has a different molecular structure and good suitability. With this kind of resin, the apparatus and the process similar to the ones used in EPS pattern manufacturing may be used to produce lost foam casting pattern and excellent castings.